Process for treating polyethylene structures and articles resulting therefrom



Allg- 1955 L. E. WOLINS 2,715,076

PRQCESS FOR TREATING FOLYETH NE UCTURES AND ARTICLES RESULTING THERE MFiled Nov. 29; 1952 Ill INVENTOR LEON E. WOLINSK I ATTORNEY UnitedStates Patent rnocnss FoR TREA'HNG .BoLrETr YLENE s'rnucrunns ANDARTICLES RESULT- mo THEREFRGM Leon E. Wolinski, Buffalo, N. Y., assignorto E. I. du Punt ale Nemours and Company, Wilmington, Del.,

This invention relates to a process of treating the surface ofpolyethylene structures and, more particularly, to a process of treatingthe surface of a polyethylene film to promote the adhesion thereto ofprinting inks and various other materials.

U. 5. Patent 2,219,700 to Perrin et al. discloses and claims apolyethylene film, i. e., a film of a solid polymer of ethylene. Ingeneral, polyethylene films are tough, semi-transparent, resistant tomany chemicals, exhibit a high degree of moisture vapor impermeability,permit the passage of oxygen, and are heat-scalable. Because of thiscombination of properties, polyethylene films are highly useful forpackaging and wrapping a great variety of materials such as chemicals,fresh produce, dried milk, textiles, hardware, etc. Probably the onlytroublesome disadvantage of polyethylene film for use in the packagingfield is the fact that standard aniline and rotogravure printing inksemployed for printing various cellulosic films, such as regeneratedcellulose and cellulose acetate films, do not adhere satisfactorily tothe surface of the film. Generally, any indicia, such as trademarks,advertising indicia, recipes, etc., imprinted upon a surface of apolyethylene film with standard oil or lacquer type inks employed forprinting cellophane film are easily smeared or rubbed off by the normalabrasions sulfered by packages during shipping, handling, etc. Hence, inorder to obtain satisfactory adhesion between a dried ink and apolyethylene film surface, it is necessary to employ a speciallycompounded ink or modify the film surface to promote improved inkadhesion.

Although printing inks compounded particularly for printing onpolyethylene films have been developed, the use of most of these inksrequires modification of standard printing processes; and the preferredapproach is treatment of the polyethylene film surface to promoteadhesion of standard oil and lacquer type inks.

An object of the present invention is to provide a process of treatingthe surface of a polyethylene structure, e. g., film, to improveadhesion of standard printing inks, i. e., promote adhesion of standardaniline and rotogravure inks employed in printing on cellophane film.Another object is to provide a process of treating the surface of apolyethylene film to improve adhesion thereof to various othermaterials, such as metals, paper, nitrocellulose coatings, and otherpolymeric coatings, e. g., nylon, polyethylene terephthalate, etc. Afurther object is to improve the adhesion of polyethylene film to itselfand other materials when using commercial adhesives. A still furtherobject is to provide a process of treating the surface of a polyethylenefilm to improve adhesion thereto of dried ink impressions and not impairthe transparency of the film. A still further object is to provide apolyethylene film having modified surface characteristics such thatdried ink imprints on the surface will not rub off when tested inaccordance with the various tests described hereinafter. Other objectswill be apparent from the following description of the invention.

These objects are realized by the present invention which, brieflystated, comprises subjecting a polyethylene structure, e. g., film, at atemperature: within the range of from about room temperature to atemperature above which substantial degradation of the polyethylenecomposition occurs, preferably from 325 C., to the action of ozone inthe presence of nitrous oxide as a reaction accelerator and, preferably,in. the presence vof ultraviolet light.

It should be understood that 325 C. is not an upper limit insofar as theoperability of the present invention is concerned. Extrusion ofpresently available polyethylene compositions alt temperaturessubstantially higher than 325 C. is not practical because the melt istoo fluid, and presently known antioxidants do not efiiciently preventdegradation of the polymer at appreciably higher temperatures. With thedevelopment of polyethylene compositions which form a more viscous meltand the discovery of more efiicient antioxidants, extrusion may becarried out more rapidly and efficiently at temperatures of 400 C. andabove, the maximum being that temperature beyond which substantialdegradation of the polyethylene composition occurs.

In the normal process of extruding molten polyethylene into film form, amolding powder or fiake of polyethylene is fed continuously into a meltextrusion machine, and the molten film continuously extruded through aslot orifice and through an air gap vertically downward into a quenchbath maintained at a temperature from 25-95 C., preferably from 30-60 C.Usually, the polyethylene is extruded from a melt maintained at atemperature Within the range from 150 to 325 C. Tubing is usually extruded from a melt at a temperature within the range from l50-2(l( C.,whereas film is extruded at a temperature within the range from 250325C. An alternative process of forming a polyethylene film comprisesmilling molten polymer on closely-spaced calender rolls to form a filmwhich is conducted vertically downward into a quench bath. in either ofthese general methods of forming a polymeric film, the space between thepoint where the molten film leaves the slot orifice or the last calenderroll and the point Where the molten film enters a quench bath willhereinafter be termed the air gap. During passage through the air gap,the film is merely permitted to pass uninhibited through the atmosphere,and this provides for some superficial cooling. Generally, the length ofthe air gap ranges from about 2" to as long as 15 in some cases.

Although it is preferred in most cases and most expedicut to treat thepolyethylene film in the air gap, i. e., at l'50325 C., the film may betreated at temperatures as low as normal room temperatures so long asthe apparatus employed provides for sufilcient time of treatmentGenerally, at lower temperatures, ultra-violet light should be employedas an accelerator along with the nitrous oxide. The apparatus employedfor treating the film at temperatures lower than 150 C. will depend onthe degree to which the film is self-supporting, i. e., in order toprevent excessive drawing or change in thickness, the film may have tobe supported during the treatment.

Because of the rapid action .of ozone upon the surface of polyethylenefilm at elevated temperatures, the process of the present invention ismost conveniently carried out by subjecting freshly extruded film at atemperature of from about 150 to about 325 C. to the action of ozone asthe film passes through the air gap. For example, freshly extruded filmmay be treated in accordance with this invention by suitably enclosingthe air gap and pro viding for the maintenance in the enclosure of agaseous atmosphere, e. g., air, and containing ozone and nitrous oxide.Provision maybe made for the employment of ultra-violet light by makingthe Walls of the enclosure transparent to ultra-violet light or byinstalling a source of ultra-violet light inside of an opaque enclosure.Hence, the process of the present invention may be carried out by makingvery simple modifications to existing film-forming or tube-formingapparatus; and, owing to the rapid action of ozone, employment of thepresent process does not preclude production of film at commerciallysatisfactory rates. Normally, in order that polythene film may betreated in accordance with the present invention as part of presentlyemployed extrusion or calendering techniques of forming the film, thetime of treatment in the air gap should be no greater than about 2seconds in order to permit operation at commercially acceptable rates.As dis cussed above, treatment at temperatures lower than 150 C. wouldrequire the use of longer treating chambers.

Preferably, the nitrous oxide may be employed in concentrationsequivalent to the mol per cent or volume per cent of ozone present, theuse of a large excess of nitrous oxide, however, not being deleterious.

For effective action within the relatively short exposure timespermissible in the treatment of film being extruded at commerciallyacceptable rates, the concentration of ozone in the treating atmospheremust be at least 0.01% by volume, of the total volume of gases presentin the treating gases. The use of ozone concentrations substantiallygreater than 5%, by volume of the total gases surrounding the film, isnot particularly practical because of the restricted capacity ofpresent-day ozone generating equipment. This applies to continuoustreatment of film wherein the ozone-containing gas, e. g., air, ispassed continuously through the treating chamber, and additional ozoneis injected into the out gases which are then recirculated. Ozoneconcentrations as high as do not tend to burn the film, i. e., impairtransparency or semitransparency of the film, so long as the time oftreatment or exposure is not excessive.

Preferably, ultra-violet light having a wave length no greater than 3900AU is employed to accelerate the action of ozone, particularly where theprocess of this invention is incorporated in the continuous productionof film.

The following examples illustrate the preferred practice of thisinvention, reference being had to the accompanying drawing wherein isshown diagrammatically an arrangement of apparatus used in carrying outthe process of this invention.

Referring to the drawing, molten polyethylene at a temperature of 265 C.was extruded in the form of a film F from extrusion hopper 1 into theair gap surrounded by an enclosure constituting a treating chamber 2,the walls of which are formed in part, at least, of transparent material(quartz glass) to provide for the transmission therethrough ofultra-violet light emitted from mercury arc lamps 3 placed two inchesfrom the film. The upper end of the chamber was closed by the extrusionhopper and the bottom of the chamber was sealed from the atmosphere byprojecting the sides thereof below the surface of the cooling water C.)in the quench bath 4. The length of the air gap was 10 inches, and thepath of travel of the film in the quench bath was also 10 inches. Air atatmospheric pressure and containing ozone and nitrous oxide was passedinto the chamber at 5 and out at 6. At no time did the temperature offilm in chamber 2 drop below 200 C. The treatment time set forth in thefollowing table is the actual time that any given increment of the filmremained in the treating chamber.

In evaluating the printability, i. e., the strength of the adhesive bondbetween the dried ink and the treated polyethylene film surface, anumber of tests were employed (5 in all); and on the basis of theresults of allof the tests, the films were rated either acceptable ornot acceptable and, if acceptable, either excellent or good. Fourdifferent inks were employed to print the treated surfaces ofpolyethylene films, and each printed sample was evaluated in accordancewith each of the five tests which will be described hereinafter. Theinks employed were as follows:

No. 1.Aniline cellophane ink (Bensing Bros. and Dceney, No. W-400).

No. 2.-Aniline polyethylene ink (Interchemical Corporation, No. PA-Red)No. 3.Rotogravure cellophane ink (Bensing Bros. and Dceney, No. G-l037).

No. 4.Rotogravure polyethylene ink (Interchemical Corporation,IN-Tag-Red, GPA Red).

In preparing the printed samples of polyethylene film, the ink wasapplied with a commercial ink spreader which comprised a steel rodhaving fine wire wrapped around the rod. The spreader produced amultiplicity of fine lines. The ink was then dried for three minutes at70 C. and thereafter permitted to cool to room temperature. Each samplewas then tested in accordance with each of the following tests, and theamount of ink rubbed off and/or removed was noted:

1. Rub Test-The inked polyethylene surface was rubbed ten times againsta hard white paper.

2. Scratch Test.The back of a fingernail was rubbed across the inkedsurface.

3. Flex Test.-The film was held between thumb and forefinger (2" apart)and flexed vigorously.

4. Pressure-Sensitive Tape Test.A pressure-sensitive tape was pressedagainst the printed surface, and then pulled off.

5. Twist Test.The printed film was folded once and then again in adirection perpendicular to the first fold. The folded ends were thentwisted once around and thereafter the film surface was examined forsmearing and/or cracking of the dried ink.

Although it is convenient and preferable to treat the polyethylene filmwith ozone and an accelerator in the air gap between the extrusionorifice and the quench bath as described above, or between the lastcalender roll and the quench bath, the film being at a temperature closeto the actual melt extrusion temperature, the process of the presentinvention may be applied to the film at a point after the film has beenquenched. To carry out the process of the present invention at thispoint would require conducting the film through a chamber wherein thefilm is maintained at a temperature not lower than room temperature andsubjecting the film therein to the action of an atmosphere containingozone and nitrous oxide.

Moreover, while the present process is employed primarily for treatingthe surface of a polyethylene film in order to produce a film which maybe successfully printed with standard oil or lacquer type inks, e. g.,aniline or rotogravure inks employed for printing on cellophane film,the present invention may be employed to modify the surface of apolyethylene film which is to be printed with inks whics are especiallymodified for printing upon a polyethylene film surface. The net resultis an even further improvement in the adhesive bonds between the driedink and the polyethylene film surface. The present invention furtherprovides for the preparation of a polyethylene film which is morereadily adherent to metals, papers, and various coatings, such as thoseof nitrocellulose; polyamides, e. g., polyhexamethylene adipamide,polyhexamethylene sebacamide, N-methoxymethyl polyhexamethyleneadipamide and other polyamides defined in U. S. P. 2,430,860, andinterpolyamides defined in U. S. P. 2,285,009; polyethyleneterephthalate; polyvinyl acetals such as polyvinyl butyral; ethylcellulose; vinyl acetatevinyl chloride copolymers; vinylidene chloridecopolymers; chlorinated rubbers; etc. Furthermore, polyethylene filmtreated by the present process is more readily adhered to itself andother base materials by using commercial adhesives, e. g., standardadhesives employed for sealing cellophane.

The process of this invention may also be employed for treating thesurface of various films fabricated from copolymers of ethylene withvarious other polymerizable materials, e. g., isobutylene, vinylacetate, styrene, vinyl chloride.

The outstanding advantage of the present process is that it provides areadily applicable and rapid method of improving the adhesion of a driedprinting ink to the surface of a polyethylene film. The process may bereadily combined with a necessary step of extruding or calenderingmolten polyethylene into film or tube form, and the additional apparatusrequired is inexpensive and easy to install.

Another outstanding advantage of the present invention is that itprovides for the preparation of an improved polyethylene film whichforms heat seals of higher bond strength than seals made with filmstreated by any other known process of improving the adhesion of printinginks to polyethylene film. This is especially true of film which hasbeen treated with a sizing composition, such as an aqueous solution ofan alkyl aryl polyglycol ether of the type defined in U. S. P.2,519,013.

As many widely different embodiments may be made Without departing fromthe spirit and scope of this invention, it is to be understood that saidinvention is in no Wise restricted except as set forth in the appendedclaims.

I claim:

1. A process for treating structures of polyethylene which comprisessubjecting said structures to the action of a gaseous atmospherecontaining at least 0.01% by volume of ozone and at least 0.01% byvolume of nitrous oxide, at a temperature within the range of from roomtemperature to the temperature beyond which substantial degradation ofthe polymer occurs, for a period of time sufiicient to render saidstructures adherent to printing ink.

2. A process for treating structures of polyethylene which comprisessubjecting said structures to the action of a gaseous atmospherecontaining at least 0.01% by volume of ozone and at least 0.01% byvolume of nitrous oxide, at a temperature within the range of from about150 to about 325 C., for a period of time sufficient to render saidstructures adherent to printing ink.

3. A process for treating structures of polyethylene which comprisessubjecting said structures to the action of a gaseous atmospherecontaining at least 0.01% by volume of ozone and at least 0.01% byvolume of nitrous oxide, in the presence of ultra-violet light having awave length no greater than 3900 AU, at a temperature within the rangeof from room temperature to the temperature beyond which substantialdegradation of the polymer occurs, for a period of time sufficient torender said structures adherent to printing ink.

4. A process for treating structures of polyethylene which comprisessubjecting said structures to the action of a gaseous atmospherecontaining at least 0.01% by volume of ozone and at least 0.01% byvolume of nitrous oxide, in the presence of ultra-violet light having awave length no greater than 3900 AU, at a temperature within the rangeof from about 150 to about 325 C., for a period of time suificient torender said structures adherent to printing ink.

5. A process for treating polyethylene film which comprises subjectingthe surface of said film to the action of a gaseous atmospherecontaining at least 0.01% by volume of ozone and at least 0.01% byvolume of nitrous oxide, at a temperature within the range of from roomtemperature to the temperature beyond which substantial degradation ofthe polymer occurs, for a period of time sutficient to render said filmsurface adherent to printing ink.

6. A polyethylene film resulting from the process of claim 5.

7. A process for treating polyethylene film which comprises subjectingthe surface of said film to the action of a gaseous atmospherecontaining at least 0.01% by volume of ozone and at least 0.01% byvolume of nitrous oxide, at a temperature within the range of from aboutto about 325 C., for a period of time sufficient to render said filmsurface adherent to printing ink.

8. A process for treating polyethylene film which comprises subjectingthe surface of said film to the action of a gaseous atmospherecontaining at least 0.01% by volume of ozone and at least 0.01% byvolume of nitrous oxide, in the presence of ultra-violet light having awave length of at least 3900 AU, at a temperature within the range offrom about 150 to about 325 C., for a period of time sufficient torender said film surface adherent to printing ink.

9. A process for treating polyethylene which comprises passingcontinuous film of polyethylene continuously through a zone wherein saidfilm is maintained at a temperature within the range of from roomtemperature to the temperature beyond which substantial degradation ofthe polyethylene occurs, and subjecting the surface of the film to theaction of a gaseous atmosphere containing at least 0.01% by volume ofozone and at least 0.01% by volume of nitrous oxide, for a period oftime sufiicient to render said surface adherent to printing ink.

10. A process for treating polyethylene which comprises passingcontinuous film of polyethylene continuously through a zone wherein saidfilm is maintained at a temperature within the range of from about 150to about 325 C., and subjecting the surface of the film to the action ofa gaseous atmosphere containing at least 0.01% by volume of ozone and atleast 0.01% by volume of nitrous oxide, for a period of time sufiicientto render said surface adherent to printing ink.

11. A process for treating polyethylene which comprises passingcontinuous film of polyethylene continuously through a zone wherein saidfilm is maintained at a temperature within the range of from about 150to about 325 C., and subjecting the surface of the film to the action ofa gaseous atmosphere containing at least 0.01% by volume of ozone and atleast 0.01% by volume of nitrous oxide, in the presence of ultra-violetlight having a wave length at least 3900 AU, for a period of timesufiicient to render said surface adherent to printing ink.

12. A process for treating polyethylene film which comprises subjectingthe surface of said film to the action of a gaseous atmospherecontaining at least 0.01% by volume of ozone and at least 0.01% byvolume of nitrous oxide, at a temperature within the range of from roomtemperature to the temperature beyond which substantial degradation ofthe polymer occurs, for a period of time sufiicient to render said filmsurface adherent to printing ink, and thereafter imprinting such surfacewith a printing ink.

References Cited in the file of this patent UNITED STATES PATENTS2,400,720 Staudinger May 21, 1946 2,499,421 Samler Mar. 7, 19502,502,841 Henderson Apr. 4, 1950 2,612,480 May Sept. 30, 1952 2,622,056De Coudres Dec. 16, 1952 2,639,998 Pavlic May 26, 1953

12. A PROCESS FOR TREATING POLYETHYLENE FILM WHICH COMPRISES SUBJECTINGTHE SURFACE OF SAID FILM TO THE ACTION OF A GASEOUS ATMOSPHERECONTAINING AT LEAST 0.01% BY VOLUME OF OZONE AND AT LEAST 0.01% BYVOLUME OF NITROUS OXIDE, AT A TEMPERATURE WITHIN THE RANGE OF FROM ROOMTEMPERATURE TO THE TEMPERATURE BEYOND WHICH SUBSTANTIAL DEGRADATION OFTHE POLYMER OCCURS, FOR A PERIOD OF TIME SUFFICIENT TO RENDER SAID FILMSURFACE ADHERENT TO PRINTING INK, AND THEREAFTER IMPRINTING SUCH SURFACEWITH A PRINTING INK.